1. Field of the Invention
The invention relates to a method and a device for detection of moving points in an image for digital television systems providing conditional-replenishment bit-rate compression in which the images received are updated or replenished only in respect of the significant points of the image which are in motion.
2. Description of the Prior Art
Methods of detection of moving points in an image for compressing the bit rate of coded television images between transmitters and receivers are already known and mostly consist in selecting data relating to the image points which have changed to an appreciable extent from one image to the next for the purpose of transmitting said data over the transmission channel which connects the transmitter to the different television receivers.
The movement-detecting devices or so-called motion detectors which are employed in the practical application of the methods mentioned above are also well-known and, in the case of each television image, carry out segmentation between the stationary and moving areas of the image.
In principle, a comparison of the difference in inter-image luminance of two similar points having a single threshold is sufficient to separate the two types of areas. In practice, however, the separation proves to be imperfect, in the first place by reason of the analog or digital noise which is always present in an image sequence and gives rise to confusion in the static or moving state of the image points and in the second place by reason of the bit-rate requirements imposed on the transmission channel.
The difficulty arises from the fact that, by modifying the adjustment of the single detection threshold of existing motion detectors, it is impossible to obtain a good rendition of the moving areas and a low bit rate in the transmission channel. In fact, by increasing the detection threshold in order to avoid detection of background noise, poorly reproduced moving areas are obtained at the level of the receivers whereas, by reducing the detection threshold in order to produce correct restitution of the movements of the image, an increasing quantity of background noise is allowed to pass, thereby increasing the transmitted data bit rate.